This invention relates to a direct current motor such as a starting motor for an engine and the like, and more specifically, the invention relates to a motor using a rotor core which has two different types of slots.
Conventional dc motors, generally, have rotor cores provided with a plurality of slots for receiving lead wires which constitute the rotor coils. These slots are circumferentially formed in the peripheral face of a rotor core so that an axially extending tooth is formed between any two adjoining slots. In a dc motor of this type, it is apparent that the power output of the motor can be increased by increasing the number of turns of the lead wire, in other words, the number of the slots. However, since increasing the number of the slots results in a decrease in the thicknesses of the teeth, the number of the slots which it is possible to form in a rotor core is limited to a predetermined value depending on the slots' widths. More specifically, the number and widths of the slots must be such that they allow each tooth of the rotor core sufficient thickness for maintaining a proper mechanical strength. Furthermore, the excess slots reduce the thicknesses of the teeth to a dimension such that an abrupt increase in the magnetic resistance of the teeth is caused, which results in no substantial increase of the power output. Use of flat lead wires with the corresponding thin slots may enable the rotor core to have more slots than that of customary round lead wires. However, the bending of the flat wires, which is required upon winding the wires around the core, is not as easy as the bending of the round wires.